The Boeing 787 Dreamliner was recently grounded across the world because its lithium ion batteries would self-combust in a blaze of glory (aka the batteries melt). What's interesting is that Boeing knew about all the battery problems in the 787 before any flight was grounded.
The NY Times reports that Air Nippon Airways, the 787's biggest operator, had replaced 10 of its batteries in the months before the battery that caused a grounding had caught fire. Boeing was notified of the battery issues (and that they were replaced) but Air Nippon Airways did not report them to safety officials because they were not required to. The Times says:
In five of the 10 replacements, All Nippon said that the main battery showed an unexpectedly low charge. An unexpected drop in a 787's main battery also occurred on the All Nippon flight that had to make an emergency landing in Japan on Jan. 16.
The airline also revealed that in three instances, the main battery failed to start normally and had to be replaced along with the charger. In other cases, one battery showed an error reading and another, used to start the auxiliary power unit, failed.
Lithium ion batteries are slightly more dangerous than the traditional batteries used in planes because it can overheat and ignite if not properly charged and discharged. It's a scary phenomenon known as thermal runway. The FAA had allowed Boeing in 2007 to start using lithium ion batteries if it could prevent such fires from happening. Looks like they were unable to in the 787 Dreamliner. [NY Times]